Reformed gasoline is an aromatics-rich gasoline, which is produced by reforming, particularly by catalytic reforming of crude oil fractions. During the reforming process, isomerizations, rearrangements, cyclizations, dehydrogenations and similar reactions take place in the alkanes and cycloalkanes contained in the petroleum or crude oil. The aromatics-rich reformed gasoline produced by catalytic reforming is an important base material for the production of aromatic compounds.
Aromatic compounds, in particular benzene, toluene, xylene and ethylbenzene are important base materials for the chemical industry, especially for the manufacture of plastics and man-made fibers. Aromatic compounds are also used as octane enhancers in gasoline. Due to the increasing demand for aromatic compounds from the chemical industry, the reaction conditions and catalysts used for catalytic reforming of crude oil fractions are designed for a high aromatics yield. As a result, however, also a higher quantity of unsaturated non-aromatics and in particular olefins, are produced.
The chemical industry requires, however, mainly pure aromatics, i.e. aromatics containing the smallest possible amount of impurities of unsaturated non-aromatics. These impurities consisting of unsaturated non-aromatics could, until now, only be separated from the aromatics by complex physical and chemical separation processes, and generally a complete removal of the non-aromatics is impossible. Bromine index and acid wash color are used as measurements for the purity level of aromatics, in particular pure benzene, and thus provide a measurement of the content of unsaturated non-aromatic impurities. According to the requirements of the chemical industry, the bromine index of pure benzene should not exceed a value of 20 and the acid wash color should not exceed a value of 1.
In a known process for separating the aromatics from reformed gasoline, an extractive distillation or a liquid--liquid extraction is initially carried out on the aromatics-containing mixture. In order to achieve the aforementioned purity levels, the aromatic fractions generated by the extraction require, however, a complex secondary treatment. Normally a secondary treatment is carried out in which the fractions are either washed with concentrated sulfuric acid or are treated with bleaching earth. Both chemical secondary processes are complex and expensive. The reaction with bleaching earth is carried out at high temperatures causing polymers to be formed which remain attached to the bleaching earth. At the same time, oligomers, leading to a relatively high acid wash color, are formed from unsaturated olefinic non-aromatics. Subsequent to the treatment with bleaching earth, a complex and costly distillation separation of pure aromatics from non-aromatics is required.